This invention relates generally to redox electrodes, and in particular to mixed ionic electronic conductor coatings to enhance the performance of redox electrodes in secondary battery cells.
The rapid proliferation of portable electronic devices in the international marketplace has led to a corresponding increase in the demand for advanced secondary batteries. The miniaturization of such devices as, for example, cellular phones, laptop computers, etc., has naturally fueled the desire for rechargeable batteries having high specific energies (light weight). At the same time, mounting concerns regarding the environmental impact of throwaway technologies, has caused a discernible shift away from primary batteries and towards rechargeable systems.
In addition, heightened awareness concerning toxic waste has motivated, in part, efforts to replace toxic cadmium electrodes in nickel/cadmium batteries with the more benign hydrogen storage electrodes in nickel/metal hydride cells. For the above reasons, there is a strong market potential for environmentally benign secondary battery technologies.
Secondary batteries are in widespread use in modern society, particularly in applications where large amounts of energy are not required. However, it is desirable to use batteries in applications requiring considerable power, and much effort has been expended in developing batteries suitable for high specific energy, medium power applications, such as for electric vehicles and load leveling. Of course, such batteries are also suitable for use in lower power applications such as cameras or portable recording devices.
At this time, the most common secondary batteries are probably the lead-acid batteries used in automobiles. These batteries have the advantage of being capable of operating for many charge cycles without significant loss of performance. However, such batteries have a low energy-to-weight ratio. Similar limitations are found in most other systems, such as Ni-Cd and nickel metal hydride systems.
Among the factors leading to the successful development of high specific energy batteries, is the fundamental need for high cell voltage and low equivalent weight electrode materials. Electrode materials must also fulfill the basic electrochemical requirements of sufficient electronic and ionic conductivity, high reversibility of the oxidation/reduction reaction, as well as excellent thermal and chemical stability within the temperature range for a particular application. Importantly, the electrode materials must be reasonably inexpensive, widely available, non-toxic, and easy to process.
Thus, a smaller, lighter, cheaper, non-toxic battery has been sought for the next generation of batteries. The low equivalent weight of lithium Tenders it attractive as a battery electrode component for improving weight ratios. Lithium provides also greater energy per volume than do the traditional battery standards, nickel and cadmium.
The low equivalent weight and low cost of sulfur and its nontoxicity renders it also an attractive candidate battery component. Successful lithium/organosulfur battery cells are known. (See, De Jonghe et al., U.S. Pat. Nos. 4,833,048 and 4,917,974; and Visco et al., U.S. Pat. No. 5,162,175.)
Recent developments in ambient-temperature sulfur electrode technology may provide commercially viable rechargeable lithium-sulfur batteries. Chu and colleagues are largely responsible for these developments which are described in U.S. Pat. Nos. 5,582,623 and 5,523,179 (issued to Chu). The patents disclose a sulfur-based positive electrode for a battery cell that has low equivalent weight and high cell voltage and consequently a high specific energy (greater than about 120 Wh/kg). The disclosed positive electrode addresses deficiencies in the prior art to provide a high capacity sulfur-based positive composite electrode suitable for use with metal (such as lithium) negative electrodes in secondary battery cells. These developments allow electrochemical utilization of elemental sulfur at levels of 50% and higher over multiple cycles. Because sulfur has a theoretical maximum capacity of 1675 milliamp-hours per gram (mAh/g) (assuming all sulfur atoms in an electrode are fully reduced during discharge), the utilization of sulfur in lithium-sulfur cells as described in the above Chu patents typically exceeds 800 mAh/g of sulfur.
The sulfur-based positive redox electrodes described in the above Chu patents provide increased capacity over previously available redox electrodes, such as in the above-noted lead-acid batteries. However, like previous battery designs, they are susceptible to precipitation of reduced end-product discharge material onto electrode current collectors. This precipitated discharge material must be removed chemically by extracting ions from the precipitate. Many conventional redox electrodes are composed of a mixture of an electrochemically active agent, an ionic conductor and an electronic conductor. Carbon-based redox electrodes, that is, electrodes that use carbon as an electronic conductor, are common. Discharge material precipitated on the electronic conductor in conventional redox electrodes is difficult to remove since ions cannot be extracted at the precipitate/electrode interface (since the electronic conductor cannot conduct ions). Instead, the chemical extraction of the ions must take place by dissolution of the precipitate into the electrolyte, a time-consuming process. The precipitate renders that part of the electrode on which it is deposited effectively inactive until the precipitate is removed. An electrode which has discharge product deposited on its surface detracting from its performance is sometimes referred to as "plugged".
An example of this situation is illustrated in FIG. 1. The figure shows a representation of the carbon-based positive electrode 100 of a lithium-sulfur secondary battery cell. One of the main components of the positive electrode is the carbon electronic conductor. As the sulfur of the positive electrode, in this case the active-sulfur compound Li.sub.2 S.sub.8, is reduced during discharge of the battery cell, as indicated by the dashed line 102 representing the discharge profile of the active-sulfur, it is gradually converted to the lower sulfides until the nonconductive discharge product Li.sub.2 S is obtained. At high concentrations near the electrode 100, Li.sub.2 S (and/or other reduced polysulfides, such as Li.sub.2 S.sub.2 or Li.sub.2 S.sub.3) comes out of solution and precipitates on the electrode surface. The Li.sub.2 S precipitate 104 plugs the electrode, preventing other chemical reactions from occurring where it forms, detracting from the electrode's performance. In order to remove the precipitate 104 from the electrode surface and restore the performance of the electrode, the Li.sub.2 S must be oxidized by extracting Li.sup.+ ions. However, where the Li.sub.2 S is precipitated on the carbon electronically conductive component of the positive electrode, ions can only be extracted from the back (electrolyte) side of the precipitate 104, since the carbon is not an ionic conductor. Removal thus relies on relatively slow dissolution of the precipitate from the back side, which is further slowed since a substantial portion of the surface area of the precipitate 104 is interfaced with the electrode surface and unavailable for dissolution.
Accordingly, a redox electrode design which mitigates this electrode plugging would enhance the performance of redox electrodes, and would thus be desirable.